Sintered tungsten-boron alloy

ABSTRACT

This application relates to a sintered tungsten-boron alloy optionally containing small amounts of certain other additives with the boron content of the alloy being less than about 0.05 weight per cent and preferably between 0.005 and 0.02 weight per cent.

United States Patent [1 1 Benesovsky [54] SINTERED TUNGSTEN-BORON ALLOY [75] Inventor: Friedrich Benesovsky, Tirol, Austria [73] Assignee: Schwarzkopf Development Corporation, New York, N.Y.

[22] Filed: Dec. 10, 1969 [21] Appl. No.: 884,053

[30] Foreign Application Priority Data Dec. 23, 1968 Austria ..l2550/68 [52] U.S. Cl. ..29/182, 75/200, 75/227 [51] Int. Cl ..B22f 1/00, C22c H00 [58] Field of Search ..75/200, 207, 224, 225, 176,

[56] References Cited UNITED STATES PATENTS 3,44l,39l 4/1969 Foldcs ..29Il82.5

[ 1 Mar. 27, 1973 l,648,679 ll/l927 Fonda ..75/207X FOREIGN PATENTS OR APPLICATIONS 1,291,894 3/1962 France ..75/176 Primary Examiner-Benjamin R. Padgett Assistant ExaminerR. E. Schafer Attorney-Morgan, Finnegan, Durham & Pine [57] ABSTRACT 15 Claims, N0 Drawings SINTERED TUNGSTEN-BORON ALLOY This application relates to a sintered alloy of tungsten containing small amounts of boron.

Sintered or arc-melted tungsten is widely used for electron tube components and in other high-temperature applications. In spite of its high melting point, tungsten recrystallizes at relatively low annealing temperatures. This property has an adverse effect on its high-temperature mechanical properties. Owing to the great hardness and the high brittle-ductile temperatures, tungsten is very difficult to shape. Even the hightemperature strength tungsten alloys, particularly those with hafnium and carbon addition, are not better in this respect. Only relatively large additions of rhenium'can lower the transition temperature markedly but, for economic reasons, rhenium being a scarce and hence expensive material, such alloys are used only for special applications.

' It has now been found that tungsten-boron alloys can be produced to have properties better than tungsten alone in that they have surprisingly low transition temperatures, higher recrystallization temperatures and a finer grain structure. Thus, the alloys of this invention can be shaped more readily than tungsten or conventional tungsten alloys and they also display greater high temperature strength.

The alloys of this invention are made in accordance with powder metallurgical techniques. It is important, however, that the boron content of the alloy be kept very low. The boron content of the alloy will be from about 0.002 to 0.05 weight per cent, preferably less than about 0.02 per cent, generally from about 0.005 to 0.02 weight per cent. In addition, the alloy should contain less than about 0.002 weight per cent of oxygen.

It is also preferred that the alloy contain a small quantity of molybdenum in the range of about 0.03 to 1 percent by weight, more preferably from about 0.05 to 0.1 percent.

A further improvement in the physical properties of the alloys of this invention can be achieved by the optional addition of one or more of zirconium and hafnium in small amounts. Generally, the presence of a total of from about 0.1 to 3 percent, preferably from about 0.05 to 0.2 percent by weight of one or both of zirconium and hafnium will be effective.

Where particularly outstanding forming properties, high temperature strength and good age hardening are desired, the alloys of the invention can contain additionally from about 0.1 to 3 percent by weight, preferably about 0.5 to l percent by weight of rhenium and from about 0.05 to 0.2 percent by weight of iron, nickel, cobalt and/or manganese.

The alloys of this invention are prepared by powder metallurgical techniques which involves the mixing together of the metallic powders, compression to form a compact of the desired shape followed by sintering and cooling.

The starting metallic powders that are employed should each have a very small oxygen content in order to insure a low oxygen content in the final alloy. Preferably, the oxygen content of the starting powders should not exceed about 350 parts per million.

Each of the starting powdered metals should have a particle size of less than about microns and preferably a particle size within the range of about 2 to 7 microns.

The boron component should preferably be added to the powder mixture in the form of a metallic boride such as tungsten boride powder rather than in the form of elemental boron. Elemental boron can, however, be used as a less desirable alternative if it is substantially free of interfering impurities. Naturally, if tungsten boride powder is used as the source of the boron, the tungsten portion of the tungsten boride will contribute to the over-all content of tungsten in the alloy and calculations must be made accordingly. Some or all of the boron can also be added in the form of other metallic borides in which the metallic component is present in small quantity and does not interfere with the results obtained.

It will often be found during operation that a certain amount of the boron will be lost during sintering, a phenomenon caused by volatilization of the boron oxides formed by reaction of boron with any residual 0xygen actually present. However, the properties of the alloy depend on the actual boron content of the end product and not on the boron content of the charged powder. The actual quantity of boron or boron-containing powder to be added and mixed with the tungsten powder to obtain a pre-determined boron content in the alloy will necessarily have to be determined by trial but the trial will be aided by knowledge of the ox ygen content of the powders and a rough calculation of the approximate quantity of boron oxides that would be expected to be vaporized. In many instances it has been found that from 30 to 100 percent excess boron must be added to the powder mixture in order to obtain the desired boron concentration in the sintered alloy.

lf molybdenum is to be added to the alloy, molybdenum powder would also be added to the powder mixture and the molybdenum powder would also have a particle size within the above-described range and again would be relatively pure and not contain any im purities in quantities sufficient to interfere with the properties of the finished alloy. Molybdenum can also be added in the form of a molybdenum-tungsten alloy or mixture.

Similarly, if zirconium and hafnium are to be added, they can be added in the form of metallic powders or in the form of zirconium hydride and hafnium powders, respectively.

Any conventional technique can be used to mix the powders together such as, for example, ball milling, or the like.

After mixing, the powder mixture is compressed into a compact of the desired shape such as, a rod or bar or particular finished article. The sintering pressure should be in the order of about 4 to 6 tons per square centimeter. The compression can be under ambient temperature conditions.

Following compression, the compacted articles are sintered at an elevated temperature within the range of about 2,500 to 2,800 C., preferably 2,600 to 2,700 C. The sintering takes place in a protective atmosphere to prevent oxide formation within the alloy. The protective atmosphere can be an atmosphere of an inert gas such as helium or argon or the like, or it can be a reducing atmosphere such as an atmosphere of hydrogen. Alternatively, sintering can take place in a protective atmosphere that is substantially evacuated by imposition of strong vacuum. Where vacuum sintering is employed, the minimum vacuum during sintering should be in the range of 10 to 10" mm of mercury. Sintering should take place for a period of time sufficient to achieve adequate alloy formation and the exact time of sintering will depend on the temperature employed, the thickness of the compact and the exact composition of the mixture. Generally, a sintering time of from about 2 to 10 hours, preferably at least 5 hours, will suffice. Depending upon the end use desired, the optimum parameters of time and temperature of sintering that are particularly desirable in any given instance can easily be determined by trial. After sintering has taken place, the sintered alloy should be allowed to cool to room temperature while remaining in a protective atmosphere.

Alloys of this invention containing zirconium and/or hafnium as well as rhenium and iron group elements can also be further heat treated by precipitation hardening in accordance with conventional techniques. The mechanical properties of such alloys, particularly their hot strength, can be greatly increased in this manner. Precipitation hardening is preferably achieved by solution annealing at the temperature in the order of between about 2,220 and 2,500 C. followed by quenching to about 1,200 to 1,800 C. and annealing at this temperature for about to 100 hours. The tungsten-boron alloys of this invention, whether or not they contain the additional additives described are useful for electron tube components and as components in X-ray and other high temperature applications in which tungsten alone is now employed.

The following examples are illustrative of the invention:

EXAMPLE 1 One hundred Grams of tungsten powder having an average particle size of 3 microns and a specific surface area of 0.5 square meters per gram was employed. This tungsten powder also had as impurities 30 ppm iron, 30 ppm silicon, 500 ppm molybdenum, 50 ppm carbon and 200 ppm oxygen. 0.25 Grams of tungsten boride were added and mixed with the tungsten powder, the tungsten boride having an average particle size of 5 microns. The powder was mixed to form an intimate mixture and then pressed into bars at a pressure of 5 tons per square centimeter. The pressed bars were then sintered for 5 hours at 2,600 C. in an atmosphere of hydrogen gas. Following sintering, the bars were allowed to cool to room temperature in the hydrogen atmosphere.

The boron content of the finished alloy was found to be about 0.01 weight per cent and was capable of being rolled, forged, drawn, etc. The product has greater high temperature strength than the comparably preferred first boron free material. In the foregoing example, the boron could have been added, if desired in the form of a ternary alloy of rhenium -ir0nboron or the iron could have been wholly or partly replaced in the ternary alloy by nickel, cobalt or manganese.

EXAMPLE 2 The procedure of Example 1 was followed except that to the powder mixture of tungsten and tungsten boride, 1 percent of hafnium was added in the form of hafnium hydride with mixing and compression into bars at a pressure of 5 tons per square centimeter. The bars were then sintered in hydrogen at 2,700 C. for 6 hours and then allowed to cool in hydrogen in the furnace. The bars were then annealed for 2 hours at 2,500 C. in hydrogen, rapidly cooled, and then again annealed for 20 hours in hydrogen at 1,600 C. Bars of excellent physical properties were achieved.

EXAMPLE 3 The method of Example 1 was followed except that in addition to tungsten and tungsten boride, additions were made of zirconium hydride and hafnium hydride in powder quantities sufficient to yield 0.5% zirconium by weight and 1.5% hafnium by weight. All other conditions are the same as in Example 1 except, in order to attain adequate diffusion of the additions into the base metal, the sintering time was extended for a period of 10 hours.

What is claimed is:

l. A sintered metal alloy consisting essentially of tungsten'and an amount of boron of less than about 0.05 percent, said alloy having a lower transition temperature, a higher recrystallization temperature, greater high temperature strength and finer grain structure than tungsten per se.

2. A sintered metal alloy as in claim 1 in which the boron content is at least about 0.002 percent by weight.

3. A sintered metal alloy as in claim 1 also containing an amount of molybdenum within the range of about 0.03 to 1 percent by weight.

4. A sintered metal alloy as in claim 2 also containing from about 0.01 to 3 percent by weight of at least one of hafnium and zirconium.

5. A sintered metal alloy consisting essentially of tungsten and from about 0.002 to 0.05 percent by weight of boron, the oxygen content of said alloy being less than about 0.002 percent by weight, said alloy having a lower transition temperature, a higher recrystallization temperature, greater high temperature strength and finer grain structure than tungsten per se.

6. A sintered metal alloy as in claim 5 in which the boron content is between about 0.005 and 0.01 percent by weight.

7. A sintered metal alloy as in claim 5 also containing from about 0.03 to 1 percent by weight of molybdenum.

8. A sintered metal alloy as in claim 5 also containing up to about 3 percent by weight of at least one of zir conium and hafnium.

9. A method of producing a tungsten-boron alloy characterized by a lower transition temperature, a higher recrystallization temperature, greater high temperature strength and finer grain structure than tungsten per se wherein said method consists essentially of mixing tungsten powder with boron or boron-containing powder, compressing the resultant mixture to form a compact, sintering the compact at a sintering tem-' perature in a protective atmosphere for a period of time and at a temperature necessary to achieve sintering, and cooling in a protective atmosphere, the quantity of boron or boron-containing powder being employed being sufficient to yield a boron content in the alloy of from about 0.002 to 0.05 percent by weight.

10. A method as in claim 9 in which the quantity of boron or boron-containing powder employed is sufficient to produce an alloy having from about 0.005 to 0.02 percent by weight of boron.

14. A method as in claim 9 in which from about 0.03

to 1 percent by weight of molybdenum is added to the powder mixture before compression.

15. A method as in claim 9 in which from about 0.05

to 3 percent by weight of at least one of zirconium and hafnium is added to the powder mixture before compression. 

2. A sintered metal alloy as in claim 1 in which the boron content is at least about 0.002 percent by weight.
 3. A sintered metal alloy as in claim 1 also containing an amount of molybdenum within the range of about 0.03 to 1 percent by weight.
 4. A sintered metal alloy as in claim 2 also containing from about 0.01 to 3 percent by weight of at least one of hafnium and zirconium.
 5. A sintered metal alloy consisting essentially of tungsten and from about 0.002 to 0.05 percent by weight of boron, the oxygen content of said alloy being less than about 0.002 percent by weight, said alloy having a lower transition temperature, a higher recrystallization temperature, greater high temperature strength and finer grain structure than tungsten per se.
 6. A sintered metal alloy as in claim 5 in which the boron content is between about 0.005 and 0.01 percent by weight.
 7. A sintered metal alloy as in claim 5 also containing from about 0.03 to 1 percent by weight of molybdenum.
 8. A sintered metal alloy as in claim 5 also containing up to about 3 percent by weight of at least one of zirconium and hafnium.
 9. A method of producing a tungsten-boron alloy characterized by a lower transition temperature, a higher recrystallization temperature, greater high temperature strength and finer grain structure than tungsten per se wherein said method consists essentially of mixing tungsten powder with boron or boron-containing powder, compressing the resultant mixture to form a compact, sintering the compact at a sintering temperature in a protective atmosphere for a period of time and at a temperature necessary to achieve sintering, and cooling in a protective atmosphere, the quantity of boron or boron-containing powder being employed being sufficient to yield a boron content in the alloy of from about 0.002 to 0.05 percent by weight.
 10. A method as in claim 9 in which the quantity of boron or boron-containing powder employed is sufficient to produce an alloy having from about 0.005 to 0.02 percent by weight of boron.
 11. A method as in claim 9 in which the oxygen content of the starting powders does not exceed about 350 parts per million.
 12. A method as in claim 11 in which the boron component is added to the powder in the form of tungsten boride.
 13. A method as in claim 12 in which the tungsten and tungsten boride powders each have a particle size in the range of about 2 to 7 microns.
 14. A method as in claim 9 in which from about 0.03 to 1 percent by weight of molybdenum is added to the powder mixture before compression.
 15. A method as in claim 9 in which from about 0.05 to 3 percent by weight of at least one of zirconium and hafnium is added to the powder mixture before compression. 